1. Field of the Invention
The invention relates to an ink droplet ejecting method and apparatus of an ink jet type.
2. Description of Related Art
According to a known ink jet printer of an ink jet type, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material, and when the flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle, while when the flow path volume increases, the ink is introduced into the ink flow path from an ink inlet. In this type of a printing head, a plurality of ink chambers are formed by partition walls of a piezoelectric ceramic material, and ink supply means, such as ink cartridges, are connected to one end of the plural ink chambers, while at the opposite end of each of the ink chambers an ink ejecting nozzle is provided (hereinafter referred to simply as "nozzles"). The partition walls are deformed in accordance with printing data to make the ink chambers smaller in volume, whereby ink droplets are ejected onto a printing medium from the nozzles to print, for example, a character or a figure.
As this type of an ink jet printer, a drop-on-demand type ink jet printer which ejects ink droplets is popular because of high ejection efficiency and low running cost. As an example of the drop-on-demand type there is known a shear mode type using a piezoelectric material, as is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051.
As shown in FIGS. 8A and 8B, this type of an ink droplet ejecting apparatus 600 comprises a bottom wall 601, a top wall 602 and shear mode actuator walls 603 located therebetween. The actuator walls 603 each comprise a lower wall 607 bonded to the bottom wall 601 and polarized in the direction of arrow 611 and an upper wall 605 formed of a piezoelectric material, the upper wall 605 being bonded to the top wall 602 and polarized in the direction of arrow 609. Adjacent actuator walls 603, as a pair, define an ink chamber 613 therebetween, and next adjacent actuator walls 603, as a pair, define a space 615 which is narrower than the ink chamber 613.
A nozzle plate 617 having nozzles 618 is fixed to one end of the ink chambers 613, while to the opposite end of the ink chambers is connected the ink supply source (not shown). On both side faces of each actuator wall 603 are formed electrodes 619, 621, respectively, as metallized layers. More specifically, the electrode 619 is formed on the actuator wall 603 on the side of the ink chamber 613, while the electrode 621 is formed on the actuator wall 603 on the side of the space 615. The surface of the electrode 619 is covered with an insulating layer 630 for insulation from ink. The electrode 621 which faces the space 615 is connected to a ground 623, and the electrode 619 provided in each ink chamber 613 is connected to a controller 625 which provides an actuator drive signal to the electrode.
The controller 625 applies a voltage to the electrode 619 in each ink chamber, whereby the associated actuator walls 603 undergo a piezoelectric thickness slip deformation in directions to increase the volume of the ink chamber 613. For example, as shown in FIG. 9, when voltage E(V) is applied to an electrode 619 c in an ink chamber 613c, electric fields are generated in directions of arrows 629, 631, 630 and 632, respectively, in actuator walls 603e, 603f, so that the actuator walls 603e, 603f undergo a piezoelectric thickness slip deformation in directions to increase the volume of the ink chamber 613 c. At this time, the internal pressure of the ink chamber 613 c, including a nozzle 618c and the vicinity thereof, decreases. The applied state of the voltage E(V) is maintained for only a one-way propagation time T of a pressure wave in the ink chamber 613. During this period, ink is supplied from the ink supply source.
The one-way propagation time T is a time required for the pressure wave in the ink chamber 613 to propagate longitudinally through the same chamber. Given that the length of the ink chamber 613 is L and the velocity of sound in the ink present in the ink chamber 613 is a, the time T is determined to be T=L/a.
According to the theory of pressure wave propagation, upon lapse of time T or an odd-multiple time thereof after the above application of voltage, the internal pressure of the ink chamber 613 reverses into a positive pressure. In conformity with this timing, the voltage being applied to the electrode 621c in the ink chamber 613c is returned to 0 (V). As a result, the actuator walls 603e and 603f revert to their original state (FIG. 8A) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure and the pressure developed by the reversion of the actuator walls 603e and 603f to their original state before the deformation are added together to afford a relatively high pressure in the vicinity of the nozzle 618c in the ink chamber 613 c, whereby an ink droplet is ejected from the nozzle 618 c. An ink supply passage 626 communicating with the ink chamber 613 is formed by members 627, 628.
In the ink droplet ejecting apparatus 600, if control lowers the driving voltage for allowing a small volume of an ink droplet to be ejected with a view to enhancing the printing resolution, there arises the problem that the speed of the ink droplet also decreases. In order that an ink droplet of a small volume may be obtained without a decrease in the ink ejection speed, there has been proposed the addition of pulses low in voltage level after application of a jet pulse and before the completion of ink ejection as disclosed in U.S. Pat. No. 4,523,200 to Howkins. In this case, a plurality of voltages are required as driving pulses and a complicated control is needed among a series of pulses, thus leading to an increase in the cost of a driver IC and of the printer.
For obtaining an ink droplet of a small volume, applicant has studied a driving method in which a non-jet pulse is applied after application of a jet pulse to an actuator. However, it turned out that if at a high temperature, continuous dot printing is performed, then the dot printing is stopped by one dot, i.e., a dot is skipped, and is thereafter started again, the ink droplet speed of the second dot after the restart decreases. This is presumed to be because the oscillating state of the ink meniscus becomes unstable due to a lowering in viscosity of the ink at the high temperature and an additional pulse is applied when the ink meniscus is retracted from the associated nozzle, thus causing a decrease of the ink droplet speed. As a result, there arises the problem that the ejected ink droplet follows a curved path and does not arrive at a correct position, causing the print quality to deteriorate.